The physical property by which threaded fasteners perform their clamping function is tension, or clamp load. This clamp load is achieved by the application of rotational force, or torque. Fastener designers rely on empirical data accumulated from tests that plot the relationship of torque vs. tension. This torque-tension relationship can be significantly affected by friction in the joint, both in the thread and at the rotated clamp surface.
Extremely high friction can cause insufficient clamp load, resulting in joint looseness and possibly shear failure of the externally threaded component. Extremely low friction can cause clamp loads that exceed the fastener's tensile load capability at assembly, resulting in stretching to the point of failure. Worse yet, the fastener could be near the tensile limit at assembly and fail in the field due to normal additional loads applied during use. To avoid potential catastrophic tensile failure, designers typically design or use fasteners that have significantly greater strength than actually required.
Friction cannot be controlled very well in the typical manufacturing environment. Consequently, assembly processes are designed to identify suspect assemblies. Installation of fasteners is typically controlled by torque with resultant torque angle being monitored to identify potential faulty assembly. However, torque angle alone is less of an indicator of clamp load than torque since a torque value is utilized as a point to begin measuring torque angle. Very high torque angle could be an indicator of high friction that could detrimentally reduce the clamp load, or some other assembly characteristic that may not affect clamp load adversely. It is necessary to evaluate assemblies with high torque angles to determine if rework is required. This is a costly and inefficient use of manufacturing resources.
Another weakness of existing threaded fasteners is the potential for inadvertent loosening of the fastened joint due to cyclic loads or vibration. A cyclic load applied to the helical surface of the thread can result in a torque force inducing a rotation in the direction opposite to the original tightening rotation.